There are a number of advantages associated with non-magnetic and non-conductive toner, if it can be utilized instead of magnetic and conductive toner. Many present electrostatic imaging technologies, such as the present ion deposition printing (MIDAX) technology, presently use magnetic and conductive toner, and therefore are limited in color and print quality, and the toners are relatively expensive. Non-magnetic and non-conductive toners are available in a wide variety of colors, are available in a smaller particle size (which enhances print quality), and are generally less expensive than existing magnetic and conductive toners. Also, non-magnetic toner images placed on a printed page are advantageous in security printing applications, and will not interfere with magnetically scanned characters on the same area of a printed document.
There have been a number of proposals for utilizing lower conductivity and magnetic strength, or non-magnetic and non-conductive, toners, such as shown in U.S. Pat. No. 4,777,106. One method, which has been recognized to effectively utilize such toners, is to have the toner maintained in a fluidized condition and electrostatically charge the toner in the fluidized state. However, prior art techniques for maintaining the fluidized bed of powdered toner have not been entirely successful, and additionally prior art systems for transferring the fluidized toner to an image cylinder or the like have been restricted in scope. For example, some such proposals are truly useful only for application to an image cylinder at or about the six o'clock position and do not control wrong polarity or low charged toner. However, there is a need, in order to develop a practical system, to apply toner to an image cylinder at or above the nine o'clock position, provide high toner transfer efficiencies, and to control wrong polarity and low charge toner in order to maintain a clean background image.
According to the present invention a method and apparatus are provided which accomplish the above mentioned goals. The basic apparatus according to the invention for applying a non-magnetic and non-conductive toner to a member containing an electrostatic pattern (typically a rotating image cylinder) comprises the following elements: A container having a closed bottom and sides, for containing powdered toner. Means for fluidizing the toner in the container. Means for simultaneously stirring and electrically charging the powdered toner in the container; and means for transferring toner from the container to the member containing an electrostatic pattern. The simultaneous stirring and electrically charging means preferably comprises a plurality of rotating elements having radially outwardly extending sharp appendages (e.g. blades) mounted within the container, and charged to a high voltage, e.g. at least about 7 kv (e.g. +8 kv), causing a coronal or atmospheric breakdown of the fluidizing gases and depositing electrical charge on the surface of the toner particles in the bed. This means, although illustrated to work in the positive polarity mode (e.g. +8 kv) and imparting a positive polarity charge on the toner, is not restricted to the positive mode only. By reversing the polarity of the rotating elements, it has been found equal performance is achieved with a negative driving potential. (The means for transferring toner to the image cylinder--e.g. the rollers--are also run in opposite polarities to those described herein and performance is equal.) The means for running in either polarity described provides a means to control the toner to run in either polarity which makes the powder (toner) used material independent. That is to say, material and surface additives used to generate a specific triboelectric charging means is independent of this described electrostatic coronal charging process.
The fluidizing means preferably comprises a gas pervious false bottom of the container, with air being introduced between a solid bottom and the false bottom to flow upwardly into the container. Also, the fluidizing means preferably comprises an electrical vibrator mounted to one closed side wall of the container.
The powdered toner supply in the container is automatically replenished whenever it drops below a desired amount. This is accomplished utilizing an optical sensing means which senses the level of the toner, and controls a slotted roller mounted at the bottom of the hopper (which has downwardly sloping side walls) to discharge more toner into the open top of the container.
The means for transferring the toner from the container to the image cylinder preferably comprises a plurality of rotating conductive metallic cylinders mounted for rotation about generally horizontal, parallel axes, and means for electrically charging the cylinders. Preferably three cylinders are provided, a first cylinder having a peripheral surface thereof mounted just above the level of toner in the container at an open top portion thereof, a second cylinder having the axis thereof mounted above the axis of rotation of the cylinder and for removal of opposite sign charged toner and low charge toner from the last cylinder, and a last cylinder having the axis of rotation thereof mounted below the axis of rotation of the first cylinder, and having the peripheral surface thereof adjacent both the peripheral surface of the first cylinder, and the image cylinder. Scrapers are preferably associated with the first and last cylinders for scraping unused toner therefrom to fall back into the container through the open top thereof.
According to another aspect of the present invention, a method of applying non-conductive and non-magnetic toner to a member having an electrostatic pattern is provided, comprising the steps of: (a) supplying powdered non-conductive and non-magnetic toner to a container having a closed bottom, closed sides, and open top; (b) simultaneously stirring and electrically charging the powdered toner in the container; (c) maintaining the powdered toner in the container fluid; and (d) transferring charged toner from the container to a member having an electrostatic pattern thereon.
According to yet another aspect of the present invention, a method of applying non-conductive and non-magnetic toner to a member having an electrostatic pattern comprising an image cylinder rotating about a generally horizontal axis, having a peripheral portion thereof exposed at or above a nine o'clock position, comprises the following steps: (a) supplying powdered non-conductive and non-magnetic toner to a container having a closed bottom, closed sides, and open top; (b) electrically charging the powdered toner in the container; (c) maintaining the powdered toner in the container fluid; and (d) transferring charged toner from the container to the exposed, approximately nine o'clock positioned, peripheral portion of the image cylinder.
It is a primary object of the present invention to provide an effective method and apparatus for application of non-magnetic and non-conductive toner to a member containing an electrostatic pattern, such as an image cylinder. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.